The present invention relates to a two-dimensional superlattice structure (so-called quantum well wire) and a three-dimensional superlattice structure (so-called quantum well box) and devices, such as a HEMT (high electron mobility transistor), diode laser and OEIC (optoelectronic integrated circuit), employing them. It also relates to a method of and apparatus for manufacturing such a device.
Heretofore, a one-dimensional hetero-superlattice structure as shown in FIG. 2 by way of example has been known.
The one-dimensional superlattice structure is so constructed that, on a substrate 201 of GaAs, several layers of GaAlAs 202, 204, 206 and 208 and several layers of GaAs 203, 205 and 207 are alternately grown in a z-direction by, e.g., molecular beam epitaxy (MBE), or chemical vapor deposition (MOCVD) or molecular beam epitaxy (MOMBE) employing an organometallic compound. In the superlattice of such a construction, conduction bands and valence bands of different levels are alternately arrayed as illustrated in FIG. 3 because the electron affinity and the energy gap differ between GaAs and GaAlAs. Thus, in case of x=0.25 by way of example, the bottom of the conduction band of Al.sub.x Ga.sub.1-x As is 0.3 eV higher than that of the conduction band of GaAs, and the top of the valence band of Al.sub.x Ga.sub.1-x As is 0.06 eV lower than that of the valence band of GaAs, thereby to form a well-type potential in which GaAs defines wells and AlGaAs defines barriers as shown in FIG. 3. Here, when the composition ratio x of Al is varied, the height of the barriers is proportional to x.
The above is the one-dimensional superlattice structure, namely, quantum well. When the barriers are sufficiently wide, electrons are confined within the potential wells and are localized. In contrast, when the barriers are narrow, electrons form small minibands (subbands) within the conduction bands and can move about within the superlattice.
With such a one-dimensional quantum well structure, a state density curve which expresses the state density D(E) of electrons versus energy becomes stepped as indicated by a solid line in FIG. 5, unlike a parabolic state density curve in the case of a bulk as shown in FIG. 4.
As stated above, the one-dimensional superlattice has the very artificial structure and exhibits the special energy density structure. In semiconductor lasers, FETs (field effect transistors), etc. employing this superlattice, therefore, excellent characteristics unattainable with a conventional structure of the bulk type have been attained.
In, for example, the semiconductor lasers, there have been achieved such excellent effects compared with the bulk type as (1) decrease in the threshold value of an injection current, (2) improvement in output stability for the temperature change, (3) increase in a gain, and (4) rise in a response rate.
Concerning such one-dimensional superlattice, two-dimensional and three-dimensional superlattice structures have been suggested as ideas, and the characteristics thereof have been theoretically computed.
The ideas of the two-dimensional and three-dimensional superlattice structures are described in, for example, "H Sakai; Japanese Journal of Applied Physics, vol. 19 (1980), No. 12, pp. L735-L738" and "Y. Arakawa and H. Sakai; Appl. Phys. Lett., vol. 40 (1982), pp. 939-941."
A two-dimensional superlattice (Quantum Well Line or Quantum Well Wire) shown in FIG. 6 is such that the foregoing structure, in which the layers of the two sorts of materials are stacked in the z-direction, is formed with periodic potential barriers 601 and 602 also in a y-direction. A three-dimensional superlattice structure (Quantum Well Box or Quantum Well Dot) shown in FIG. 7 is such that, besides periodic potential barriers 701 and 702 in the y-direction, periodic potential barriers 711 and 712 are formed in an x-direction.
Here, the potential barriers 601, 602, 701, 702, 711, 712 . . . etc. in the x-direction and y-direction need not be stacked into the alternate layers of the different materials GaAs and GaAlAs as in the z-direction, but they may well be formed by making spatial gaps.
State density curves corresponding to the two-dimensional and three-dimensional superlattices in FIGS. 6 and 7 become as illustrated in FIGS. 8 and 9, respectively. From the respective curves, there are seen situations in which electrons are localized in one dimension and zero dimension, whereby the state densities thereof concentrate in still smaller regions of energy. It is therefore predicted that, in semiconductor lasers employing these superlattices, the features (1)-(4) as mentioned above will be further improved.
For such two-dimensional and three-dimensional superlattice structures, however, there has hitherto been no suitable method of manufacture. This is because there has been no means to form narrow alternate potential wells and barriers in x- and y- directions onto the superlattice having z- directional laminated structure.
As one of few experimental examples reported, the attempt by Petroff et al is illustrated in FIGS. 10a-10d. This is described in "Physics and Application of Semiconductor Superlattice" edited by the Physical Society of Japan, pp. 85-87 (1984) or "P. M. Petroff et al; Appl. Phys. Lett.," 41 (1982), pp 635-638. In this example, a one-dimensional superlattice, in which Ga.sub.0.75 Al.sub.0.25 As 1002, GaAs 1003, Ga.sub.0.75 Al.sub.0.25 As 1004, GaAs 1005, Ga.sub.0.75 Al.sub.0.25 As 1006 and GaAs 1007 are grown in alternate fashion on a GaAs substrate 1001 by an expedient (or means) such as molecular beam epitaxy, has its uppermost layer coated with a photoresist, which is exposed to light and then developed using a mask formed by an expedient such as electron beam lithography, whereby a stripe-like resist layer 1008 having a width of about 2 .mu.m is left as shown in FIG. 10a. Here, the lengthwise direction of the stripe is perpendicular to the sheet of the drawing.
Thereafter, when the resulting structure is subjected to chemical etching, even a part under the resist layer is obliquely etched as indicated by numerals 1009 and 1010 in FIG. 10b owing to the presence of the stripe-like resist layer. The etching can be ended so as to finally leave the materials of trapezoidal or triangular section 1011, 1012 as shown in FIG. 10c. Thereafter, the remaining resist 1008 is stripped off, and a film 1013 of Al.sub.0.31 Ga.sub.0.69 As having a great energy gap is formed for protection on the whole surface of the resulting structure as shown in FIG. 10d by molecular beam epitaxy or the like.
The structure thus manufactured becomes a multilayer wire structure in which the lateral dimensions of the respective layers in its section are different. The sectional dimensions of the quantum well wire 1014 of the uppermost layer can be made about 200 .ANG..times.200 .ANG., of which the quantum effect of the two-dimensional superlattice can be expected. The measurement of cathode luminescence at a low temperature (about 20.degree. K.) has revealed that, as shown in FIG. 11, besides a peak 1101 from the original one-dimensional superlattice, the peak 1102 of new luminescence based on the two-dimensional quantum effect is obtained on the shorter wavelength side.
However, the prior-art method of manufacturing the two-dimensional superlattice (quantum well wire) by Petroff et al as stated above has had the following disadvantages:
(1) With a quantum well wire whose respective layers have the same width, the peculiar potential as shown in FIG. 8 appears clearly owing to the effect of periodicity. In contrast, since the Petroff et al two-dimensional superlattice has the triangular section, the widths of the respective layers differ from one another, so that the effect of periodicity is difficult to develop, and the essential characteristics of the two-dimensional quantum well wire are difficult to appear.
(2) Although the oblique etching under the resist layer is employed, the control of the sectional shape with this method is considerably difficult, and hence, the control of the layer widths of the quantum well wire is difficult.
(3) By repeating such patterns in the lateral direction, a multi-quantum well wire can be formed. However, it is difficult to render the intervals of the patterns less than several .mu.m, and the effect of multiplicity (periodicity) is difficult to develop.
(4) In order to vary the pitch of the superlattice, the space between the neighboring grooves thereof, the groove width thereof, and the depth thereof (in the z-direction) as desired in accordance with a purpose or intended characteristics, tools including a mask pattern etc. need to be remade, and much labor and a long time are expended.
(5) Alternatively, the pattern can be formed without employing the mask and by exposing the resist layer with direct electron beam lithography. In this case, the pitch, space, groove width etc. of the superlattice as mentioned in Item (4) can be varied comparatively easily. Even in this case, however, the processes of resist patterning and etching are employed, so that the control of the above parameters at high precision is difficult, and much labor is required.
Besides, regarding the manufacture of the two-dimensional or three-dimensional quantum well, it is possible to mention the techniques of the official gazettes of Japanese Patent Applications Laid-open No. 250684/1985 and No. 222190/1986 employing an etching process, No. 42481/1987, No. 36886/1987 and No. 108592/1987 employing ion-implantation by a focused ion beam, and No. 89383/1987 employing a special expedient for molecular beam epitaxy.